How to create procedural surface defects with Cycles?
The simplest way is with a bump map. You can use a mixture of various procedural noise textures and a Bump node to create one quickly:



It's up to you to make come up with a texture that you like. I recommend this talk for some tips on generating realistic patterns with procedural textures.





This probably isn't necessary in your case, but if you want the bump mapping to affect the actual geometry you can use the Displacement output with Experimental Features enabled. See What is the difference between the displace socket and a bump map?